The invention relates to a thermal joint for transferring heat from one object to another. The invention also relates to a method of transferring heat between a heat-generating component, such as a semiconductor device, and a heat-removing component, such as a heat sink.
There are numerous devices which require efficient heat transfer. For example, high speed computers and other fast electronic systems often require assemblies of many integrated circuit chips in which each chip contains many active devices, and many chips are spaced closely together. During normal operation, the devices dissipate a very large power density, especially bipolar transistor devices. Proper electronic operation of the devices necessitates a relatively cool operating temperature which, in turn, requires adequate cooling of the power density. Conversely, the maximum allowable operation temperature of the device and integrated circuit chip in combination with the limited cooling capacity presently available limit the allowable power density, circuit density, and system speed. Improved device and integrated circuit chip cooling would result in increased permissible power density, circuit density, and system speed.
Whenever it is desired to transfer or dissipate heat from one object to another by conduction, an important component in the exchange is the interfacial joint between the two objects. A typical method of enhancing the heat exchange between two objects is to machine the interface surfaces of the two objects to high tolerances and hold them tightly together so that a relatively high percentage of the joining surface areas are in contact. Not only is this method expensive, but the thermal efficiency of the joint is a function of the smoothness of the surface finish and the amount of pressure applied thereacross. Even though the surface irregularities are microscopic and the pressure employed between the two contacting surfaces large, only relatively low thermal efficiency results. This is because the interface region between the two objects includes relatively limited contact regions surrounded by air gaps and voids. The presence of air gaps and voids in the interface region tends to increase the thermal resistance of the interface.
The thermal resistance of an interface between two objects can be reduced by providing an interface material which fills the air gaps and voids in the joining surfaces. Several materials, ranging from thermal grease to phase-transition compounds, have been used in some in cases, typically in electronic device cooling, as a contact material. A main function of these interstitial materials is to minimize the number of voids and to increase the contact area either by viscous flow upon application of contact pressure or by chemical reaction. However, these contact materials are largely based on compositions with relatively poor thermal conductivity, such as amorphous grease/oil, silicon grease, and paraffin wax. As such, these contact materials offer only limited gain in thermal performance. In an operating environment which experiences high heat flux or high operating temperatures, the performance of these materials as a thermal joint may rapidly deteriorate due to the loss of volatile constituents. To alleviate such problems, these materials are sometimes mixed with a thermally conductive powder or combined with a metallic frame. However, such designs lead to complexity and increased costs.
Brazing or soldering has been considered as another way to reduce the thermal resistance of an interface between two objects. Although brazing a metallic interface between two objects may improve the thermal contact, its application is limited only to the surface that can react with the soldering material. For non-reactive surfaces, such as aluminum, ceramic materials, and plastics, deposition of a bond layer (typically noble metals) should be preceded before soldering or brazing. However, fracture of the brazed material may occur due to thermal mismatch between joining bodies or deposited layers. Factors, such as relatively high costs and difficulty of rework, may limit the application of soldering or brazing in forming an efficient thermal joint.
For the foregoing reasons, there is a need for a thermal joint which would offer relatively high thermal conductivity. Preferably, the thermal conductivity of the thermal joint is comparable to typical metals or their alloys. It would be more desirable if the thermal joint involved minimum complexity so that it can be easily implemented.
The aforementioned need is met by one or more aspects of the invention. In one aspect, the invention relates to a thermal joint for facilitating heat transfer. The thermal joint includes an alloy of at least two constituents; the alloy has an operating temperature TP, a liquid temperature TL, and a solid temperature TS; when the operating temperature TP falls between the liquid temperature TL and the solid temperature TS, the alloy includes at least a liquid phase and at least a solid phase. Preferably, the liquid phase is substantially in equilibrium with the solid phase. The thermal joint is so dimensioned to be positioned between a heat-generating component and a heat-dissipating component.
In another aspect, the invention relates a device that includes the thermal joint positioned between a heat-generating device and a heat-removing device. Such a device may be incorporated in various apparatuses, including but not limited to, computers, printers, cellular phones, television sets, video cassette recorders, power amplifiers, transformers, power supply circuits, high-power transistors, diodes, and switching boards.
In still another aspect, the invention relates to a method of dissipating heat. The method include one or more of the following steps: (a) providing a thermal joint comprising an alloy of at least two constituents between a heat-generating component and a heat-dissipating component; and (b) operating the thermal joint at a temperature between the liquid temperature TL and the solid temperature TS of the alloy so that the alloy includes at least a liquid phase and at least a solid phase. Preferably, the liquid phase is substantially in equilibrium with the solid phase.
In yet another aspect, the invention relates to method of making a thermal joint with low thermal resistance. The method includes one or more of the following steps: (a) providing an alloy of at least two constituents which has a liquid temperature TL and a solid temperature TS; (b) placing the alloy between a heat-generating component and a heat-dissipating component to define at least one interface having interfacial voids; (c) subjecting the alloy to an operating temperature above the solid temperature TS to generate a liquid phase to fill the interfacial voids; and (d) lowering the operating temperature below the liquid temperature TL.
Additional aspects of the invention, advantages, and objects of the invention become apparent with the following description.